Various processes can be used to produce polyethylene, including gas phase processes, solution processes, and slurry processes. In ethylene slurry polymerization processes, diluents such as hexane or isobutane may be used to dissolve the ethylene monomer, comonomers and hydrogen, and the monomer(s) are polymerized with a catalyst. Following polymerization, the polymer product formed is present as slurry of solid polyethylene particles suspended in the liquid medium. The individual length of the obtained polyethylene chains is not always identical but the produced molecules have a statistical distribution of their chain length, including extremely low molecular weight and oligomeric products. These undesired byproducts of the ethylene polymerization reactions, such as waxes, have a high solubility in the liquid phase and remain, at least partially, dissolved in the suspension medium.
In exemplary multi-reactor cascade processes, shown e.g., in WIPO Pat. App. Pub. No. WO 2005/077992 A1 or WIPO Pat. App. Pub. No. WO 2012/028591 A1, the reactors can be operated in parallel or in series, and the types and amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal or multimodal polyethylene material. Such multimodal compositions are used in a variety of applications; e.g., WIPO Pat. App. Pub. No. WO 2012/069400 A1 discloses trimodal polyethylene compositions for blow moldings.
Slurry polymerization systems for ethylene can use external slurry coolers in a reactor recirculation loop to remove heat produced in the ethylene polymerization reactions. Caused by a reduced solubility at lower temperature, low molecular weight waxes, which may be produced in the reactors as a byproduct, can solidify when the slurry flows through the cooler and contacts the cold walls of the heat exchangers. This may foul the heat exchangers, i.e. may cause a build-up of a wax layer on the cold walls of the heat exchangers, decreasing the heat transfer in the heat exchanger and reducing their efficiency in heat removal.
Various methods have been disclosed for reducing heat exchanger fouling. One method involves the injection of various chemicals into the process stream; e.g. European Pat. App. No. EP 1645569 A1 discloses using a nonionic surfactant in a hydrocarbon-containing gas stream in a petrochemical or polyolefin production plant. Another method for reducing heat exchanger fouling involves the use of electrical/mechanical means. For example, WIPO Pat. App. Pub. No. WO 2004/094319 A1 discloses using a direct current electric potential on the wall of a pipe or duct. WIPO Pat. App. Pub. No. WO 2007/136697 A2 and WIPO Pat. App. Pub. No. WO 2008/002423 A2 disclose using mechanical vibrations in the heat exchanger. However for ethylene polymerization processes, injection of chemicals into the process stream could negatively affect polymer product properties, and using direct current or mechanical vibrations may not be practical.
Disclosed herein are ethylene-slurry-polymerization processes operating heat exchanger which have an improved heat removal performance.